Laser diodes are extensively used in optical transmission systems as light emitters for transmitting data in digital form over telecommunications networks. Optical transmission is based around generating a constant optical signal regardless of changes in the laser diode's operating environmental conditions or ageing considerations resultant from the progress of time.
The transmission is achieved by analysis of the laser's optical power output/current input characteristic curve, which has an initial inactive linear portion which is almost horizontal with a very slight upward slope. The curve then has a knee portion which is generally called the threshold point at which stimulated light emission commences. The characteristic curve then continues as a relatively linear portion with a relatively steep slope. This, is referred to as the linear operating portion or operating region of the slope, and is the usual operating region for optical transmission systems.
A typical optical transmitter 100 includes a laser diode 200, a monitor photo diode 300 and a laser diode driver 400 and is shown in FIG. 1. The laser diode 200/laser diode driver 400 combination is factory calibrated to a particular average power Pav, and optical modulation amplitude (OMA). Two currents are controlled by the laser diode driver 400. A DC bias current is applied to the laser diode of value sufficient to maintain a predetermined average optical power output. A switched modulation current is applied to the laser diode 200 to establish a predetermined modulation amplitude OMA.
When transmitting data the modulation current is switched by the data stream. This data generally has an equal probability of ones and zeros. The only measurement taken from the laser diode is the monitor photo diode (MPD) current. The MPD gives a current proportional to the laser output power. Generally, the MPD bandwidth is much lower than the data rate so that the MPD current is a measure of the average laser output power. Typically the data rate is about 2.5 Gbps, and the MPD bandwidth is about 1 MHz. A tuning of each laser diode is required as the threshold current (the current at which the characteristic curve passes the threshold point), the post threshold current to light slope efficiency LI, optical coupling from the laser diode to the monitor photo diode and monitor photo diode responsivity changes between devices.
FIG. 2 shows the transfer function of an ideal laser diode. The power/current characteristic curve tends to vary with temperature and also varies as the laser diode ages. Three typical power/current characteristic curves of a laser diode, namely, curves A, B and C are illustrated in FIG. 2. The curves A, B and C illustrate how the power/current characteristic curves vary with temperature. The curve A illustrates the power/current characteristic curve of the laser diode when operating at 0° C. The curve B illustrates the characteristic curve of the laser diode operating at 25° C., while the curve C illustrates the characteristic curve of the laser diode operating at 70° C. An operating range of 0° C. to 70° C. is not an unusual operating temperature range for a laser diode. In each curve A, B and C the inactive portion is illustrated by the letter e. The threshold point of the curve is illustrated by the letter f, while the operating portion of each curve is illustrated by the letter g. The effect of ageing is somewhat similar to that of increasing temperature.
The input current in milliamps (mA) to the laser diode is plotted on the X axis of the graph of FIG. 2 while the optical power output in milliwatts (mW) of the laser diode is plotted on the Y axis. The bias current to the laser diode is indicated as Ib, while the modulation current is indicated as Im. The power output of the laser diode when the bias current Ib and the modulation current Im are applied to the laser diode is indicated as P1 while the power output of the laser diode when the only current applied to the laser diode is the bias current Ib is indicated as P0. The average power output of the laser diode is indicated as Pav which, assuming an equal number of digital ones and zeros in the data stream, is equal to half the sum of P1 and P0. It will be appreciated that if the laser operates too close to the threshold point, that the wavelength of the light changes and the switching speed may slow down. In order for the laser diode to operate efficiently, the bias current should be sufficient to operate the laser diode in the linear operating portion of the power/current characteristic curve just above the threshold point, in other words, the point h of the curve B, for example. To achieve this the bias current operates the laser diode to produce a power output of P0. In this way, when the modulation current Im is applied to the laser diode on top of the bias current Ib the laser diode operates in the linear operating portion of the curve, namely, between the point h and the point k on the curve B. By operating the laser diode so that the power output varies between the points h and k on the characteristic curve B in response to the modulation current the laser diode operates with an optimum OMA, which will be understood as the difference between the power output P1 and the power output P0. It is sometimes also common to define the ratio of the power output P1 to the power output P0 as the extinction ratio of the laser diode, which is a term often used in combination with the average power to define the performance of the laser. It will be appreciated that with a control of the OMA and the average power, that an equivalent control of the extinction ratio is effected. Another term commonly used with reference to laser diode operation is the slope or slope efficiency (light/current) LI. This term is used to represent the average slope in the operating region.
However, it will be clear from the curves A, B and C of FIG. 2 that should the operating temperature of the laser diode vary, unless the bias current Ib and the modulation current Im are varied to compensate for the change in operating temperature, the laser diode will operate incorrectly. For example, if the bias current Ib and the modulation current Im were set to operate the laser diode at an operating temperature of 25° C., an increase in the operating temperature would immediately cause the OMA of the laser diode to drop, and also would result in a reduction in the average power output Pav of the laser diode.
Accordingly, in order for a laser diode to provide an adequate average power Pav and OMA over its life and over a typical range of operating temperatures, control circuitry is required for altering the bias current Ib and the modulation Im to compensate for changes in operating temperature and as the laser diode ages.
In short reach applications (applications over a short fibre length) direct modulation of the laser diode is used to meet price and size requirements. These interfaces have traditionally used a combination of open and closed loop control systems.
An open loop approach is based upon measuring environmental conditions and adjusting control parameters and is based on the assumption that every laser reacts in the same manner. A typical open loop control circuit is shown in FIG. 3, which shows a closed loop control of average power and open loop control of OMA. The circuit can be considered as comprising an open loop portion and a closed loop portion. The closed loop portion comprises a variable resistor Rpset 500 at a potential between the current output of the MPD 300, and a voltage Vee. The current output from the MPD is converted to a voltage Vpset and the control loop incorporating summing element 600 and a reference voltage source 700 are adapted to keep the average power constant by keeping Vpset and hence the MPD current constant. A constant MPD current then equates to a constant average power at the laser, thus effecting a control of the average power. It will be appreciated that errors in the average optical power will be present due to MPD tracking errors and control loop effects such as the temperature coefficient of Rpset and the input offset voltage of the control loop input stage.
The open loop control of the OMA effects control of the OMA by incorporating a temperature dependant element 800 to monitor the operating temperature of the laser diode. The modulation current is then altered in response to temperature change of the laser diode. The temperature modified modulation current is then modulated by a data stream fed into the current through a data switch 900, and used to drive the laser diode 200.
A disadvantage of such circuits is that they tend to be inaccurate. They do not measure the OMA directly. Measuring temperature gives only an indirect measure of the slope efficiency LI, and is not particularly accurate as the LI as discussed may, in general, drift with age. Thus, any corrections made to correct the OMA based on the operating temperature of the laser diode may be incorrect, thus leading to incorrect operation of the laser diode. The method also assumes that all lasers behave identically with regard to temperature which is not the actual situation. The simplification is sustainable in small or low volumes as the maintenance considerations can be dealt with, and the impaired performance is acceptable as part of a price/performance compromise. As the volumes increase, the compromises cost more in terms of human resources required to maintain a design and support structure.
These early systems have been modified to new monolithic control systems that actively measure the characteristics of the laser and adjust the drive currents accordingly. Circuits performing these functions have been available for some time and an example is shown in FIG. 4. This circuit utilises a concept known as dual loop control, a term used to define a circuit that controls both the bias current and the modulation current based on measurements taken directly from the laser diode. There are various implementations used by dual loop controllers. Some use peak detection to determine the OMA. Peak detection is limited in that the response of the monitor photo diode is usually not sufficiently quick to allow detection of P1 and P0 in the data stream. This can be overcome by relying on several consecutive ones and zeroes in various header formats. This, however, can make the device application and data rate specific as well as placing speed requirements on the monitor photo diode.
Alternative dual loop implementations, such as that disclosed in U.S. Pat. No. 5,850,409 assigned to Maxim, are independent of data rate or monitor photo diode bandwidth and operate on the principle of the addition of a low frequency small signal into the optical waveform on top of the modulation current. A simplified example of the operation of such a dual loop scheme using tone control is shown in FIG. 5. A low frequency tone 10 is added to the modulation current and can be considered as sitting on top of the optical one level. It will be understood that the frequency of this tonal element is typically much less than the frequency of the modulation current and is low enough to be detected directly by the MPD. In this example, the control loop has a measure of the average power via the monitor photo diode current and the laser modulation amplitude via the tone amplitude imposed in the monitor photo diode current. By referring to FIG. 5, it can be seen that from the similar triangles rule that (impd1′−impd1)/Δimod=(impd1−impd0)/imod. If Δimod=imod/K, i.e. proportional to imod, then it can be seen that (impd1−impd0)=k*(impd1′−impd1). Since impd1 and impd0 are a measure of P1 and P0, it will be understood that (impd1′−impd1) is proportional to the OMA. Due to the low bandwidth of the MPD, impd1 and impd1′ are not directly observed at the MPD output. What is observed is the change in the average MPD current impd_av′−impd_av which is equal to (impd1′−impd1)/2. Therefore, the amplitude of the tone in the average MPD current is proportional to the OMA and can be used as a control parameter to control the OMA. Therefore it will be appreciated that the control circuitry can maintain a constant average power and OMA by controlling the bias current and the modulation current, and has many advantages over open loop systems including the following:    laser diode slope efficiency variation over temperature is tracked directly not inferred in some open loop manner,    laser diode slope efficiency variation due to ageing is directly tracked over time    statistical sampling of laser diode slope efficiency temperature coefficient is eliminated from the design process    on-going statistical sampling of laser diode slope efficiency temperature coefficient is eliminated in manufacturing    second sourcing options are greatly enhanced as one is not limited to specific components for inclusion in the circuitry.
All these features have allowed for more robust optical systems to be manufactured but are still limited in that they rely on the assumption that the operating region of the laser diode is linear. FIG. 6 shows a comparison of theoretical and actual slope efficiency and it can be seen that the actual operating region of the laser diode is in effect not linear. It will be understood that the measurement of OMA by application of a test signal at one point can lead to errors as the estimate can be too large or too little depending on the position at which the test signal was applied.
There, therefore, exists a need for a method and circuit that is based on an analysis of the laser diode power output and the fact that non-linearities are present, and takes the existence of the non-linearities into consideration when providing a measurement value for OMA.